This invention relates to a method and apparatus for manufacturing coaxial cable of the type wherein a plastic insulating material is extruded and wrapped around the outer periphery of an inner conductor of copper, aluminum, etc. to form one or more spiral spacing ribs, and a tubular insulating sheath is simultaneously formed on the outer periphery of the rib.
In conventional coaxial cables various types of insulating layers have been proposed to obtain low dielectric constant or permittivity and enhance mechanical strength. In one type of such cable a polyethylene cord or ribbon having a rectangular cross-section is spirally wound around the inner conductor to form a helical spacing rib and an insulating sheath is thereafter formed around the rib by extruding a polyethylene tube thereover, or alternatively the spirally formed rib and surrounding tubular sheath may be simultaneously formed on the inner conductor by concurrent plastic extrusions.
The present invention relates to an improvement of the latter conventional method. According to such method the insulating layer is obtained by extruding a molten plastic material onto the travelling inner conductor to form a spiral rib, and the concurrently extruded outer sheath is melt bonded to the ridge portion of the rib. As illustrated in FIG. 1, an inner conductor 1 supplied from a supply bobbin 3 is passed through reforming rollers 4 to remove any curl or set due to the bobbin winding, and the linear conductor is thereafter fed through a preheater 5 to a dual extruder 6 at which an insulating layer 2 consisting of a spirally wound rib 2a and a surrounding tubular sheath 2b is formed on the conductor, as illustrated in FIG. 2. The insulated cable core is then passed through a vacuum sizing device 7 to obtain a uniform diameter through its length and through a water pool 8 to coolingly solidify it.
The rib 2a initially extends outwardly in a radial direction immediately after passing through the sizing device 7, as seen in FIG. 3(a). According to this conventional method, however, as the extrusion speed or conductor feed speed increases the rib gradually becomes curved during the solidification process as seen in FIG. 3(b), and upon final completion of the solidification the rib is further curved as seen in FIG. 3(c). This phenomenon is explained as follows. In the simultaneous extrusion of the molten plastic material to form the rib and the sheath, the root portion of the rib is melt bonded to the inner conductor and the ridge portion thereof to the inner periphery of the sheath. Since the sheath is radially outwardly expanded when it passes through the vacuum sizing device to control its outer diameter, no rib curvature occurs. During the subsequent cooling process, however, the tubular sheath thermally contracts in a longitudinal direction to a greater degree than the inner conductor, and the tensile stress exerted on the rib root thereby causes the curvature seen in FIGS. 3(b) and 3(c). Such a warped rib insulating layer as shown in FIG. 3(c) does not possess high mechanical strength and uniformity throughout its length, and a coaxial cable employing such an insulating layer has poor high frequency transmission and mechanical strength characteristics.